In general, it is required of this type of the compressor to reduce the rising torque at startup, to prevent abnormal pressure increase encountered in compressing the liquid, and to improve the quality of sealing in the compression chambers. In order to meet these requirements, it has been proposed to have the scroll portion of the movable scroll member slightly spaced apart from the scroll portion of the fixed scroll member during standstill, and to increase the amount of offset or the radius of offset rotation of the movable scroll member at startup by resorting to the centrifugal force or the pressure prevailing in the compression chamber, for contacting the movable and fixed scroll portions with each other for improving the sealing of the sealed or compression chambers.
A practical example is shown in FIG. 7 wherein a cylindrical sleeve 31 is loosely fitted radially movably on an offset pin 11 secured to the inner end of a rotary shaft 7, and a flange member 32 secured to the movable scroll member is mounted on the sleeve 31 by way of a radial needle bearing 17. An annular space defined by the outer surface of the offset pin 11 and the inner surface of the cylindrical sleeve 31 is divided by a pair of sealing members 33 into a pair of semiarcuate gaps 105, 106. The pressure in the compression chamber is conducted into one gap 105 through a pressure conduction chamber 34 and an air bleed opening 35 of the offset pin 11 for displacing the sleeve 31 outwards or away from the rotary shaft 7 so as to increase the radius of offset rotation of the movable scroll member (See Japanese Provisional Patent Publication No. 37521/1980).
With such a device, the offset pin 11 and the sleeve 31 are held to each other by sealing members 33. It is now supposed that the offset pin 11 has been subjected to a transverse force as a result of rotation thereof. Then, with a high pressure acting in the gap 105 as shown in FIG. 8, the central axis 0.sub.3 of the sleeve 31 (movable scroll member) may be deviated from a straight line l connecting a central axis 0.sub.1 of the rotary shaft 7 and a central axis 0.sub.2 of the offset pin 11, resulting in difficulties involved in elevating the contact pressure between the scroll portions. If the gap 105 is enlarged for avoiding these difficulties, the axis 0.sub.3 may be deviated further away from the straight line l and the mating relation between the offset pin 11 and the sleeve 31 may be affected more markedly. Hence, the gap 105 may not be enlarged beyond a certain value.
A further device of the type in which the radius of offset rotation of the movable scroll member is changed is shown in FIG. 9, wherein a slider 37 is reciprocably fitted in a slider groove 36 of the rotary shaft 7 and a boss 38 of a movable scroll member is supported by the slider 37. The slider 37 is urged by a coil spring 39 in a direction to reduce the radius of offset rotation of the movable scroll member. Upon initiation of compression, the oil stored in the housing may be introduced into the slider groove 36 via offset opening 40 in the rotary shaft 7 and pressurized under the centrifugal force of the rotary shaft for shifting the slider 37 in a direction to increase the radius of offset rotation of the movable scroll member.
However, when the compressor shown in FIG. 9 is used for a vehicle air conditioner, for example, the oil pressure acting in the slider groove 36 via offset opening 40 may be changed severely with fluctuations in the number of revolutions per minute of the vehicle engine, thus affecting the sealing force and the performance of compression.